In his Introduction to Social Psychology.
 The reasons of their
irresistible strength have been
explained in a most brilliant manner by Dr. Peisker in the
first volume of the ‘Cambridge Medieval History.’
The numbers of every species are determined, not by the procreative power of its members, which always greatly exceeds the capacity of the earth to support a progeny increasing in geometrical progression, but by two factors, the activity of its enemies and the available supply of food. Those species which survive owe their success in the struggle for existence mainly to one of two qualities, enormous fertility or parental care. The female cod spawns about 6,000,000 eggs at a time, of which at most one-third—perhaps much less—are afterwards fertilised. An infinitesimal proportion of these escapes being devoured by fish or fowl. An insect-eating bird is said to require for its support about 250,000 insects a year, and the number of such birds must amount to thousands of millions. As a rule there is a kind of equilibrium between the forces of destruction and of reproduction. If a species is nearly exterminated by its enemies, those enemies lose their food-supply and perish themselves. In some sheltered spot the survivors of the victims remain and increase till they begin to send out colonies again. In some species, such as the mice in La Plata, and the beasts and birds which devour them, there is an alternation of increase and decrease, to be accounted for in this way. But permanent disturbances of equilibrium sometimes occur. The rabbit in Australia, having found a virgin soil, multiplied for some time almost up to the limit of its natural fertility and is firmly established on that continent. The brown rat (some say) has exterminated our black rat and the Maori rat in New Zealand. The microbe of the terrible disease which the crews of Columbus brought back to